The continued development of metal-oxide-semiconductor field-effect transistors (MOSFET) has improved the speed, density, and cost per unit function of integrated circuits. One way to improve transistor performance is through selective application of stress to the transistor channel region. Stress distorts or strains the semiconductor crystal lattice, which affects the band alignment and charge transport properties of the semiconductor. By controlling the magnitude and distribution of stress in a finished device, manufacturers can increase carrier mobility and improve device performance. There are several existing approaches of introducing stress in the transistor channel region.
In U.S. Pat. No. 7,494,884, assigned to Taiwan Semiconductor Manufacturing Co., Ltd., MOS transistors have localized stressors for improving carrier mobility. A gate electrode is formed over a substrate. A carrier channel region is provided in the substrate under the gate electrode. Source/drain regions are provided on each side of the carrier channel region. The source/drain regions include an embedded stressor having a lattice spacing different from the substrate. The substrate is silicon and the embedded stressor is SiGe (for PMOS) or SiC (for NMOS). An epitaxy process that includes using HCl gas selectively forms a stressor layer within the crystalline source/drain regions and not on polycrystalline regions of the structure. In a PMOS transistor, the embedded SiGe stressor applies a compressive strain to channel region. In an NMOS transistor, the embedded stressor comprises SiC, and it applies a tensile strain to the transistor channel region.
The conventional process taught by U.S. Pat. No. 7,494,884 forms sidewall spacers on opposite sides of the gate electrode and gate dielectric. The sidewall spacers serve as self aligning masks for performing one or more ion implants within the source/drain regions. The embedded stressor regions are then positioned on either side of the sidewall spacers, and are thus separated from the channel.
An improved method for forming a device with an embedded stressor is desired.